This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of shelf life and activity of the catalysts employed in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by a carbonylation reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst may be Group VIIIB metal compound, i.e., a compound of a metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum.
Further developments in the carbonylation reaction, including the use of palladium(II) acetate in combination with such cocatalysts as cobalt pentadentate complexes and terpyridines, are disclosed in U.S. Pat. Nos. 5,231,210, 5,284,964 and 5,399,734. These patents also disclose the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package. The use of hexaalkylguanidinium chlorides or bromides in place of the quaternary salts is disclosed in copending provisional application Ser. No. 60/40,300.
The use of such Group VIIIB metal salts as the acetates is accompanied by certain disadvantages relating to the shelf life and activity of the catalyst mixture. It would be convenient to store the catalyst as a feed solution in the reactant hydroxyaromatic compound, said feed solution including the Group VIIIB metal salt in combination with at least one and preferably all of the other catalyst components. However, such feed solutions have extremely limited shelf life since the Group VIIIB metal precipitates therefrom on storage.
It would also be convenient to recycle the catalyst after use. Typically, the diaryl carbonate is recovered as an adduct with the hydroxyaromatic compound leaving the catalyst constituents in solution in further hydroxyaromatic compound. As noted herein-above, however, such solutions have very short shelf life.
Finally, temperatures for the carbonylation reaction on the order of 60-150.degree. C. are disclosed in the aforementioned patents. It is generally found, however, that the divalent Group VIIIB metal acetates such as palladium(II) acetate afford diary carbonate in high yield only at temperatures in excess of about 100.degree. C. Therefore, it would be desirable to employ a catalyst having a high degree of activity over a wider range of temperatures, including temperatures substantially below 100.degree. C.